fn validate_page_references(
document: &Document,
labels: &BTreeMap<String, LabelTarget>,
diagnostics: &mut Vec<Diagnostic>,
)Expand description
Report an undeclared label in a @page(label) reference as MOS0033,
mirroring the @label cross-reference check. A page reference resolves to a
page number later, through the layout fixpoint (issue #72), but an unknown
label is a lower-time error exactly like a bad @ref, and catching it
here means mos check reports it without needing to lay the document out.